Electrical drives are supplied nowadays from switched power sections which are usually driven with pulse-width-modulated signals (PWM signals). For this purpose, power converters are used which for each phase of the electrical machine to be controlled have a power output controlled via at least two semiconductor switches connected in a half-bridge circuit. In the prior art, there are various possibilities for generating the driving signals of the semiconductor switches, e.g. outputting a PWM signal with a fixed switching frequency with standard components using digital technology. For this purpose, control words having a word width of a plurality of bits can be written to the components. The digital logic generates therefrom the pulse-width-modulated driving signals for the semiconductor switches. What is disadvantageous here is that, on account of the synchronous logic, the switching patterns can be changed only before two points in time at the beginning and in the middle of the switching frequency period.
Proposals for controlling electrical machines are given e.g. by DE 10 2012 206 323 A1 or the thesis by Jonathan Bernard Bradshaw, “Bit-Streams Control of Doubly Fed Induction Generators”, Mar. 7, 2012, University of Auckland, New Zealand.
A further disadvantage of known power converters for controlling electrical machines is that the PWM signals are generated with a fixed switching frequency that is usually in the range perceptible to human beings. Therefore, the operation of the electrical machine is then accompanied by a frequently disturbing, uniform secondary noise, e.g. high-frequency whistling. If the switching frequency is put outside the audible range, increased switching losses arise.
A further disadvantage of known power converters is that the pulse width modulation is effected in an open loop in the customary digital embodiment. The driving signals for the semiconductor switches are transmitted to the power section and, as a result of diverse effects, the voltage-time integrals actually present at the load, i.e. at the electrical machine, differ from the setpoint values at the input of the power converter. In this regard, the magnitude of the supply voltage present at the power section, influenced by fluctuations resulting from residues of the input AC voltage from the rectifier, voltage raising during generator operation of the load, etc., is incorporated multiplicatively to a full level. The prior art attempts to take account of such effects through measurements and in the form of feedforward switching in the modulation. An additional factor is that the semiconductor switches have voltage drops as soon as they carry current. For active switches and in diodes, said voltage drops are generally different and dependent on the chip temperature. Such effects cannot readily be compensated for in feedforward switching with fixed values. Significant voltage errors arise as a result of the dead times which have to be inserted between the switch-off of one semiconductor switch of a half-bridge and the switch-on of the other semiconductor switch of the half-bridge following the calculation of the switching times, in order to prevent short circuits of the DC supply. Depending on the current sign, the sign of the absent voltage-time integral changes as well. This leads to considerable nonlinear distortions of the effective voltages generated and can be compensated for only in part by feedforward control with a current-dependent table since the actual switching times of the semiconductor switches, which are temperature-dependent and subject to manufacturing fluctuations, are incorporated into the offset voltages. In addition, the switching edges of the voltage are not ideal, but rather form voltage ramps, the gradient of which is in turn dependent on the current magnitude. This effect, too, is incorporated into the output voltage as an error.